B Newton's Rings central spot brightness

AI Thread Summary
When light is illuminated from below a plano-convex lens, the central spot appears bright due to constructive interference, while illumination from above results in a dark central spot due to a phase change from reflection. The difference in brightness is attributed to the refractive index of the materials involved, where light refracted through glass experiences a pi phase shift. The presence of reflection at the interface between two identical materials, like glass, affects the intensity of transmission, with no reflection leading to higher transmission intensity. The discussion highlights the interplay of light behavior and material properties in optical phenomena. Understanding these principles is essential for applications in optics and lens design.
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How come when the illumination is from below, the central spot is bright, but when the illumination is from above, the central spot is dark?
 
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Do you know what causes the rings?
 
I think when shining light from underneath, whether a central bright fringe is obtained or not also depends on the thickness/height of the plano-convex lens.

For the light above case, at the centre, the 2 rays travel the same distance, only that one is reflected off a medium (air) of lower refractive index, so no phase change. The other is refracted off a medium of higher refractive index (glass), so there is a pi phase change, leading to a central dark spot
 
Ibix said:
Do you know what causes the rings?
Hi sir, could you please explain why shining light from underneath produces a bright spot at the centre?
 
phantomvommand said:
Hi sir, could you please explain why shining light from underneath produces a bright spot at the centre?
Let's say you are on the other side of the glass from the light. The center of the curved and straight glasses touch. Will you get any reflection at the interface (hint: is it really an interface if two identical materials are in direct contact)?

If you get reflection, what effect does this have on the intensity of transmission? If you get no reflection, what effect does this have on the intensity of transmission?
 
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Ibix said:
Let's say you are on the other side of the glass from the light. The center of the curved and straight glasses touch. Will you get any reflection at the interface (hint: is it really an interface if two identical materials are in direct contact)?
This is an example of why I love PF so much. Just a few words and no equations to give pretty much the whole story!
 

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